The invention is directed generally to control electronics and, more particularly, to an analog phase locked loop wherein the natural frequency and damping factors are digitally controlled.
Phase locked loops are well known in the art as analog systems that automatically track a frequency fluctuating input signal. Referring to FIG. 1, a general description of an embodiment of the invention is shown. The phase locked loop includes a voltage controlled oscillator (VCO) 12 whose frequency is controlled by the voltage V.sub.c which is the filtered voltage from a phase detector 10. This causes a VCO frequency to move toward the input signal V.sub.i. The voltage output V.sub.c is available for communications applications, such as receivers, tone detectors and other well-known applications for phase locked loops.
A phase locked loop is characterized by two factors, the natural frequency (.omega..sub.n) and damping (Zeta). In any application, it is desirable to adapt the performance of the phase locked loop to changing conditions. This could be done by controlling .omega..sub.n and Zeta. This is particularly advantageous when trying to recover a signal which may be buried in noise. The invention provides such control of .omega..sub.n and Zeta. In one example of an embodiment of the invention, an 8-bit digital-to-analog converter is used yielding a control range of 256 to 1 for each factor.